1. Technical Field
The invention relates to a dental tool for preparing a tooth for a dental crown.
2. Background Information
Various tools are available for the preparation of a tooth by machining, the effectiveness of such tools being based on differing functional methods. Files having elongated and flat processing bodies have become known that have an abrasive working face on one broad side and, given an oscillating movement with amplitudes directed in the longitudinal direction of the file, are suitable for surface processing of the tooth. The magnitude of the amplitudes of movement is considerable and generally amounts to several millimeters. A cavity in the tooth cannot be worked with such a file. This applies in particular to a cavity having a basal surface transverse to the direction of working. With respect to such a basal surface the file is not only ineffective but the basal surface interferes with the movement of the file, so that damage to the file and breakage thereof would occur if it were to be used for the preparation of such a cavity.
In WO 96/14024 a tool is described having an abrasive processing section which is effective laterally and frontally and which is suitable, with an oscillating drive in the sense of a vibration which is transmitted to the tool by a dental handpiece, for preparing a lateral cavity in a tooth. With this known design the processing section is a processing head which is thickened with respect to the tool shank and which has a cross-sectional shape converging towards its free end. On the side located opposite the lateral processing section the tool has a smooth face. As a result, the known tool is suitable for working a cavity in the approximal region of a tooth. In the course of this working procedure the processing section is preferably lowered from the occlusal region towards the cervical region. In this connection the cavity may have already been worked beforehand by means of a rotary tool or it may also be prepared with the processing section without preliminary work. In the process the adjacent tooth remains unimpaired, since the smooth face located opposite the processing section does not damage the adjacent tooth.
The abrasiveness of the effective working faces of the processing section is created by means of a studding of juxtaposed small grains of hard material, preferably of diamond. In functional operation the small hard grains prove to be a plurality of cutting edges, the working faces that are present in each case being effective over substantially their entire area.
The amplitudes of the oscillating movements of the processing section in functional operation may be executed by three-dimensional, circular or elliptical movements in the sense of a vibration. In order to avoid jamming in the course of extracting the tool from the cavity, the processing section has a cross-sectional shape that is convergent towards its free end.
With this known design the tool has the working face on its side facing the tooth, on its long sides extending transverse thereto and on its front side, said working face extending continuously in the region of these sides.
Since, on account of the short-stroke vibrational drive with an existing tool, on the one hand the preparatory capacity is comparatively low and on the other hand in the case of a crown preparation a relatively large amount of tooth substance has to be worked out, it is advantageous to prepare a crown preparation with a conventional rotating tool in such a way that a tooth stump is formed having a lateral free space which is filled out with the crown to be mounted on top.
It is moreover known that the edge region between the circumferential surface of a tooth and the face pertaining to the marginal region of the crown preparation that runs into the circumferential surface is sensitive and has a tendency to break off. This is to be ascribed to the fact that the dental enamel has a prismatic structure, the prisms being bounded by imaginary radial lines emanating from the centre of the tooth. In this connection the tooth may be imagined as a truncated egg or a truncated cone which is arranged in the gum in countersunk manner. With a view to preventing the dental enamel from breaking off in the region of the edge described above, it has already been proposed to break the edge through the provision of a chamfer and to fill out this chamfer with a marginal web disposed on the free margin of the crown in such a way that the free lateral face of the tooth and the free lateral face of the crown pass into one another in stepless manner.